A Long Term Evolution (LTE) network makes use of orthogonal frequency-division multiplexing (ODFM) in the downlink, and DFT-spread ODFM in the uplink. A simple illustration of the LTE downlink physical resource can therefore be seen as a time-frequency grid as shown in FIG. 1. Each resource element corresponds to one OFDM subcarrier during one ODFM symbol interval. In the time domain, LTE downlink transmissions are organised into radio frames of 10 ms, each consisting of 10 subframes of 1 ms.
Resource allocation in LTE is typically described in terms of time-frequency radio resource units called resource blocks (RB) or physical resource blocks (PRB). Each resource block corresponds to one slot of 0.5 ms in the time domain, and 12 contiguous subcarriers in the frequency domain. All of the PRBs of two time-consecutive slots constitute one subframe. A pair of two time-adjacent resource blocks is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting at 0 from one end of the system bandwidth.
Downlink transmissions are scheduled dynamically, with the basestation transmitting information in each subframe regarding to which terminals and on which resource blocks data is transmitted for that subframe. This control signalling is typically transmitted in the first 1 to 3 ODFM symbols in each subframe, and the number of symbols used is known as the control format indicator (CFI). The CFI is indicated by the physical CFI channel (PCHICH) transmitted in the first symbol of the control region. The control region also contains physical downlink control channels (PDCCH) and may also contain physical HARQ indication channels (PHICH) carrying ACK/NACK messages for the uplink transmission. The remaining ODFM symbols in the subframe are denoted the shared data channel region, and contain the shared data channel (PDSCH).
The downlink subframe also contains common reference symbols (CRD), which are used for coherent demodulation of (e.g.) the control information. A downlink system with CFI=3 ODFM symbols as control is shown in FIG. 3.
The PDCCH is used to carry downlink control information (DCI), for example scheduling decision and power-control commands. More particularly, the DCI comprises:                Downlink scheduling assignments, including PDSCH resource indication, transport format, hybrid-ARQ information, and control information for spatial multiplexing (where applicable). A downlink scheduling assignment also includes power control commands for the physical uplink control channel (PUCCH) used to transmit hybrid-ARQ acknowledgements in response to downlink scheduling assignments.        Uplink scheduling grants, which include information relating to PUSCH resource indication, transport formal, and hybrid-ARQ. An uplink scheduling grant also includes a command for power control of the PUSCH.        Power control commands for a group of terminals. These commands are complementary to those included in the scheduling assignments/grants.        
Each PDCCH carries a single DCI message in one of the formats above. Since multiple terminals may be scheduled simultaneously (on both downlink and uplink) there must be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on a separate PDCCH, and consequently there are typically multiple simultaneous PDCCH transmissions within each cell. In order to support different radio-channel conditions, link adaptation can be used, in which the code rate of the PDCCH is selected to match the radio channel conditions.
Interference between cells of a mobile network can have a severe negative impact on radio performance where cells overlap. This is a particular problem in very dense networks, or in heterogeneous networks (“het nets”, described below) utilising small cells (e.g. micro cells, pico cells, femto cells). The interference may be mitigated by the use of techniques such as inter-cell interference coordination (ICIC), in which physical resource blocks (PRBs) in one cell are muted in a coordinated manner, so that the signal to interference and noise ratio (SINR) is improved for users registered to adjacent cells, located near the cell border, and scheduled in those PRBs. Unfortunately, since such ICIC schemes require some of the available transmission resources of the cell to be left idle, they reduce the available bandwidth for transmission of the cell.
Heterogeneous network have recently gained large interest within the mobile cellular industry, and are regarded by many operators and vendors as necessary in order to meet high user expectation in mobile broadband applications. Heterogeneous networks can be characterised an deployments made up of a mixture of differently sized and overlapping cells. An example of such a network where pico cells are deployed within the coverage area of a macro cell is illustrated in FIG. 4. A pico cell is a much smaller basestation transmitting with low output power, and typically covers a much smaller (e.g. by a few orders of magnitude) area than a macro cell (base station).
Heterogeneous networks represent an alternative to creating denser cellular networks, and have previously been considered in cellular networks as a way of relieving traffic from the macro cells in regions of high traffic. This improves both the capacity and throughput of the macro cell, by offloading traffic from the larger cell. The throughput of users in the pico cell is also improved, as they are closer to their serving cell. This latter effect is now being exploited in mobile broadband applications, by providing low-power cells merely to improve the data rates of users in those locations, rather than for load balancing.
User equipments (UEs) making use of the cellular network constantly monitor which cell they should be associated with. This monitoring is typically conducted by evaluating the radio reception quality of the serving cell (i.e. the cell the UE is currently associated with) and comparing it to the radio reception quality of other cells. In the radio reception quality of a neighbouring cell is better than that of the serving call, the UE will establish itself on the neighbouring cell in order to ensure the best service for the user. In LTE networks, the procedures for changing cell association depend on which of the two RRC states (RRC_IDLE and RRC_CONNECTED) the user equipment is in. When connected, the UE is known by the radio access network (RAN) and cell association decisions are taken by the RAN, generally based on mobility measurement reports by the UE. If a mobility measurement report indicates that the UE is better served by a neighbouring cell, then the network initiates a handover procedure. Mobility measurement reports contain the measured reference signal received power (RSRP) or reference signal received quality (RSRQ), both of which are measured in dB.
Depending on how the mobility measurements are used, and whether a configurable offset is included, a UE may be connected to the cell with the strongest RSRP, or the best path gain, or some combination of the two. The different cell association principles do not typically result in the same cell being selected when the base station output powers differ. This is known as link imbalance, and is illustrated in FIG. 5. For example, the output power of a pico base station is on the order of 30 dBm, while a macro basestation may have an output power of 46 dBm. As a consequence, the RSRP of the macro cell may be greater than that of the pico cell even in the vicinity of the pico cell. For downlink transmission, it is better to select a cell based on the received power, whereas for uplink transmission, it is better to select the cell with the least path loss. Therefore, it may be beneficial to connect to the pico cell even if the macro downlink is much stronger.
Increasing the coverage of small cells for operation in link imbalance zones can be done (for example) by adding a cell selection offset or bias to the RSRP measurements. However, operations with larger offsets or handover biases require ICIC across layers, particularly in highly loaded systems, to prevent signals from the pico cell from being swamped in the link imbalance zone.
In LTE Rel-10 networks, enhanced ICIC has been devised, for use with RSRP offsets of up to 6 dB. In enhanced ICIC (eICIC), the physical downlink shared channel (PDSCH) in a cell is muted or transmitted on reduced power for an entire subframe. Such almost blank subframes (ABSs) protect cell-edge users served by small, low power nodes (e.g. pico nodes) from interference from the local macro cell. However, the capacity of the macro cell is significantly degraded due to the blanking of subframes to protect the pico cell. According to Rel-11 (further enhanced ICIC), the throughput of a UE in the macro cell is reduced, as it cannot be scheduled (or must transmit at lower power) during the ABSs.
A user equipment receiving data must first detect physical layer control information broadcast by the cell in order to know which resource blocks contain the data intended for that UE, as well as other information required to demodulate the received data. The timing of the downlink data is generally not known in advance, so the UE must monitor the physical layer control transmissions in all subframes.
The principle of time-domain ICIC is illustrated in FIG. 6. In this case, a macro cell creates an ABS by avoiding scheduling data to users of the macro cell in certain subframes. This creates protected radio resources for pico cells within the macro cell. The macro cell indicates the location of the ABSs to the pico cells via the LTE backhaul X2 interface. The pico cell can then take this information into account when scheduling users operating within the link imbalance zone, prioritising these users into the protected subframes. Users operating close to the pico cell may be scheduled in all subframes, since the signal strength of the pico cell will be much greater than that of the macro cell in this region. Time domain ICIC requires that the pico cells are synchronised with the macro cell, in order to ensure that the subframes overlap properly.
In LTE Rel-10, the transmission power of the PDSCH within an ABS is strictly set to zero. This has been relaxed in LTE Rel-11, which allows for the transmit power of certain subframes to be reduced by some dB, while the CRS remains at full power. One example is reduced power subframes (RPS), which are supported by transmission mode 10 in LTE Rel-11.
Transmission mode 10 (TM10) has two main features, improved interference estimation, and the possibility for more flexible PDSCH transmission from different nodes in the network. TM10 is scheduled using DCI format 2D, which contains 2 “PQI” bits. [PQI is an abbreviation of “PDSCH to RE mapping and quasi co-location assumption indicator”]. These bits select one out of for RRC configured PDSCH to RE mapping and quasi co-location states. Each state describes how the PDSCH should be mapped to the RE in the particular schedules subframe, i.e. which RE should be excluded in the PDSCH to RE mapping within a PRB pair, such as the location of common reference signals (CRS), channel state information reference signals (CSI-RS) and from which signal in the subframe the legacy control channel mappings (PCFICH, PDCCH and PHICH) ends and the PDSCH mapping should start. Each of the RRC sets configured for the UE contains a PDSCH antenna port quasi-co-located with a CSI-RS signal. The UE may assume that the CSI-RS is being transmitted from the same node as the PDSCH, and may therefore use the CSI-RS to estimate channel properties such as Doppler shift, Doppler spread, delay spread, and average delay. This information is then used to aid in demodulating the PDSCH.
TM10 allows the UE to be dynamically scheduled from up to 4 different nodes, without the need to perform a handover. The node used for transmitting/receiving is determined for each subframe. This requires each of the nodes to be transmitting a CSI-RS signal that is orthogonal to the CSI-RS used by each of the other nodes. The different nodes may be different eNodeBs, which may have different CRS patterns, or they could be physically separate radio heads within the same eNodeB or cell. They may also be different sectors of a site. The networks configures the UE with a state for each of the CSI-RS signals. The UE then estimates the channel properties for each node from the corresponding CSI-RS signal, and uses them to demodulate the corresponding PDSCH.
When scheduling the UE on the PDSCH using DCI format 2D, the network uses the PQI bits to indicate which state the PDSCH is using to that the UE knows the correct PDSCH to RE mapping, and which channel properties should be assumed for demodulation.
In Rel-11, the enhanced control channel (EPDCCH) was introduced. The EPDCCH is similar to a PDSCH transmission, in that it is mapped as a PDSCH to the whole subframe in a set of 2, 4 or 8 PRB pairs which are configured for a specific UE. The EPDCCH contains the UE specific search space, which is used to schedule PDSCH and PUSCH transmissions to/from the UE. Two EPDCCH sets can be configured for each UE, each containing 2, 4, or 8 PRB pairs. When the UE is operating in TM10, each of the two sets is an RRC specifically configured for the UE, and associated with one of the four TM10 transmission states. This allows the network to perform dynamic node selection by associating each EPDCCH set with a node and the configured state for that node when transmitting scheduling control information to the UE. Since only two of the four possible TM10 transmission states are available, this provides less flexibility than node switching based on PDSCH.
TM10 is also used for dynamic point switching (DPS). In this use case, the network switches transmission dynamically between two or more cooperating nodes. This switching may occur as fast as the transmission time interval (TTI), which is 1 ms. When PDSCH transmission takes place from one of the selected nodes, the scheduled resources (i.e. PRB pairs) are not scheduled in the other nodes in the cooperating set of nodes. Therefore, the network can select which node to use based on the channel quality and/or traffic load in each of the nodes. In order to use DPS, a fast backhaul is required between each of the nodes and a centralised scheduler. DPS may also be used in combination with EPDCCH switching for two nodes, as discussed above.
As can be seen from the above description, interference coordination causes a loss in network capacity, as some resources on the macro node must be left idle to reduce interference on the pico node. In particular, the use of ABS or RBS forces the macro BS not to transmit (or to transmit at reduced power) for entire subframes, which reduces the overall capacity of the macro node. It is therefore desirable that macro cells making use of ABS and RPS can be exploited as efficiently as possible by the radio network.